ChatOpenRouter Leaks Httpx.AsyncClient Instances Causing Ephemeral Port Exhaustion

In `_build_client()`, the method instantiates `httpx.AsyncClient` (and `httpx.Client`) every time `ChatOpenRouter` is created, but the class lacks `close()`, `aclose()`, `__del__`, or context manager support to release these resources. The default `app_url` and `app_title` parameters trigger `extra_headers` logic, ensuring new clients are always created.

Run the provided reproduction script: instantiate 100 ChatOpenRouter objects in a loop without explicitly closing them; monitor open connections with psutil; resources accumulate because no teardown is called.

Closing the client while other coroutines are still using the same instance may cause RuntimeError or premature connection termination. In multi-threaded or concurrent async tasks, use a shared client or synchronize access; avoid calling __del__ as it is not guaranteed to run and may cause issues during interpreter shutdown. Rollback: if this fix introduces inconsistent client states, revert to previous behavior and advise users to pass a shared httpx.AsyncClient or use a global pool.